1. Field of the Invention
The present disclosure generally relates to image processing and, more particularly, to halftone processing for converting input image data into image data having fewer gradations.
2. Description of the Related Art
In recent years, printers are widely used to print images generated by computers, images captured by digital cameras, and images read by scanners. Image data handled by a computer typically has more gradations than a printer can output on a recording medium. Halftone processing is applied to digital image data to reduce the number of gradations so that an output apparatus such as a printer can output the gradations.
One of the known methods for halftone processing is expressing pseudo gradations in each unit area (cell). As a method for expressing pseudo gradations cell by cell, Japanese Patent Application Laid-Open No. 2007-194904 discusses a technique for calculating a barycentric position in each cell of point symmetric shape based on the pixel values of pixels corresponding to the cell, and growing dots from the barycentric position.
Japanese Patent Application Laid-Open No. 2007-82090 discusses a method for simply outputting the pixel values of pixels in a unit area if the unit area includes only white pixels and/or black pixels. If not, an average of the pixel values of the pixels in the unit area is output as the output values of the respective pixels.
According to the technique discussed in Japanese Patent Application Laid-Open No. 2007-194904, if a cell includes a plurality of edges or thin lines, the barycentric position of the dots may shift to one side and fail to be set in an appropriate position. In such a case, the resulting image has low resolution due to blurred edges or thin lines.
According to the technique discussed in Japanese Patent Application Laid-Open No. 2007-82090, a unit area may include a pixel having a pixel value expressing a halftone other than white and black pixels. Even if such a unit area includes an edge, the edge will not be detected. As a result, averaged data is output with a drop in resolution.